1 88 THE POPULAR SCIENCE MONTHLY 



there are few subjects in experimental physiology that are more difficult 

 of study. Notwithstanding that most of us doubtless believe that we 

 know the symptoms of nervous fatigue well, physiologists have been 

 able to discover only scraps of fact in this field. The isolated nerve of 

 a cold-blooded animal, when artificially stimulated in the laboratory, 

 can perform its work for many hours without showing the least sign 

 of a diminution of power. Only when placed under unfavorable con- 

 ditions, such as in light anesthesia, or when deprived of oxygen, does 

 the nerve exhibit with continued stimulation a gradual loss of con- 

 ductivity. From this it is inferred that the nerve fiber itself under 

 normal circumstances is highly resistant to fatigue, and that any 

 unfavorable dissimilative changes which it undergoes in activity are 

 compensated for at once by an equal assimilation. This highly interest- 

 ing and suggestive conclusion is perhaps equally true of nerve centers. 

 Hodge and others, it is true, have demonstrated morphological changes 

 in nerve cells as the result of artificial stimulation and of normal daily 

 activity. Thus the nuclei of the brain cells of a honey bee may show a 

 diminution in volume of 75 per cent., at the end of a day's labor; and 

 the English sparrow, though popularly regarded as less typical of in- 

 dustry, reveals almost as much cerebral activity. Notwithstanding 

 these evidences of metabolism, no one has yet succeeded in obtaining, by 

 direct or reflex artificial stimulation of the nerve ganglia, the spinal cord 

 or the brain of animals, indisputable physiological evidence of the 

 genuine fatigue of the nerve structures involved. Many attempts have 

 been made to detect fatigue in the nervous system by testing the 

 muscular power, as by the employment of the ergograph, the instrument 

 in which a muscle or set of muscles is made to perform a series of volun- 

 tary contractions and lift a given weight, the progress of fatigue being 

 indicated by the rate at which the lift diminishes. But endeavors to 

 arrive at an exact analysis of the result of such an apparently simple 

 experiment have given rise to a controversy as to the location of the 

 fatigue, some investigators claiming it for the muscles, others for 

 the brain. 



A still further attempt at the investigation of brain fatigue is 

 through the study of certain mental processes during or following long- 

 continued effort. Mental fatigue is characterized by a diminution of 

 attention, a difficulty in concentrating one's thoughts, slowness in react- 

 ing to sensory stimuli, in memorizing or in reasoning, difficulty in 

 recalling memorized passages, errors or slowness in mathematical cal- 

 culations, and other phenomena. While these are obvious in the fa- 

 tigued individual, all attempts at exact measurement of them and the 

 deduction therefrom of the degree of psychical or physical fatigue have 

 failed. 



Thus, while some of the characteristics of nervous fatigue are 

 known, all methods heretofore adopted to study the fatigue of the- 



